In recent years, techniques of binding nucleic acid-binding protein factors elucidated through various analyses to an intended sequence have been established, and they are coming to be used. Use of this sequence-specific binding is enabling analysis of intracellular localization of a target nucleic acid (DNA or RNA), elimination of a target DNA sequence, or expression control (activation or inactivation) of a protein-encoding gene existing downstream of a target DNA sequence.
There are being conducted researches and developments using the zinc finger protein (Non-patent documents 1 and 2), TAL effecter (TALE, Non-patent document 3, Patent document 1), and CRISPR (Non-patent documents 4 and 5) as protein factors that act on DNA as materials for protein engineering. However, types of such protein factors are still extremely limited.
For example, the artificial enzyme, zinc finger nuclease (ZFN), known as an artificial DNA-cleaving enzyme, is a chimera protein obtained by binding a part that is constituted by linking 3 to 6 zinc fingers that specifically recognize a DNA consisting of 3 or 4 nucleotides and bind to it, and recognizes a nucleotide sequence in a sequence unit of 3 or 4 nucleotides with one DNA cleavage domain of a bacterial DNA-cleaving enzyme (for example, FokI) (Non-patent document 2). In such a chimera protein, the zinc finger domain is a protein domain that is known to bind to DNA, and it is based on the knowledge that many transcription factors have the aforementioned domain, and bind to a specific DNA sequence to control expression of a gene. By using two of ZFNs each having three zinc fingers, cleavage of one site per 70 billion nucleotides can be induced in theory.
However, because of the high cost required for the production of ZFNs, etc., the methods using ZFNs have not come to be widely used yet. Moreover, functional sorting efficiency of ZFNs is bad, and it is suggested that the methods have a problem also in this respect. Furthermore, since a zinc finger domain consisting of n of zinc fingers tends to recognize a sequence of (GNN)n, the methods also have a problem that degree of freedom for the target gene sequence is low.
An artificial enzyme, TALEN, has also been developed by binding a protein consisting of a combinatory sequence of module parts that can recognize every one nucleotide, TAL effecter (TALE), with a DNA cleavage domain of a bacterial DNA-cleaving enzyme (for example, Fold), and it is being investigated as an artificial enzyme that can replace ZFNs (Non-patent document 3). This TALEN is an enzyme generated by fusing a DNA binding domain of a transcription factor of a plant pathogenic Xanthomonas bacterium, and the DNA cleavage domain of the DNA restriction enzyme Fold, and it is known to bind to a neighboring DNA sequence to form a dimer and cleave a double strand DNA. Since, as for this molecule, the DNA binding domain of TALE found from a bacterium that infects with plants recognize one base with a combination of amino acids at two sites in the TALE motif consisting of 34 amino acid residues, it has a characteristic that binding property for a target DNA can be chosen by choosing the repetitive structure of the TALE module. TALEN using the DNA binding domain that has such a characteristic as mentioned above has a characteristic that it enables introduction of mutation into a target gene, like ZFNs, but the significant superiority thereof to ZFNs is that degree of freedom for the target gene (nucleotide sequence) is markedly improved, and the nucleotide to which it binds can be defined with a code.
However, since the total conformation of TALEN has not been elucidated, the DNA cleavage site of TALEN has not been identified at present. Therefore, it has a problem that cleavage site of TALEN is inaccurate, and is not fixed, compared with ZFNs, and it also cleaves even a similar sequence. Therefore, it has a problem that a nucleotide sequence cannot be accurately cleaved at an intended target site with a DNA-cleaving enzyme. For these reasons, it is desired to develop and provide a novel artificial DNA-cleaving enzyme free from the aforementioned problems.
On the basis of genome sequence information, PPR proteins (proteins having a pentatricopeptide repeat (PPR) motif) constituting a big family of no less than 500 members only for plants have been identified (Non-patent document 6). The PPR proteins are nucleus-encoded proteins, but are known to act on or involved in control, cleavage, translation, splicing, RNA edition, and RNA stability chiefly at an RNA level in organelles (chloroplasts and mitochondria) in a gene-specific manner. The PPR proteins typically have a structure consisting of about 10 contiguous 35-amino acid motifs of low conservativeness, i.e., PPR motifs, and it is considered that the combination of the PPR motifs is responsible for the sequence-selective binding with RNA. Almost all the PPR proteins consist only of repetition of about 10 PPR motifs, and any domain required for exhibiting a catalytic action is not found in many cases. Therefore, it is considered that the PPR proteins are essentially RNA adapters (Non-patent document 7).
In general, binding of a protein and DNA, and binding of a protein and RNA are attained by different molecular mechanisms. Therefore, a DNA-binding protein generally does not bind to RNA, whereas an RNA-binding protein generally does not bind to DNA. For example, in the case of the pumilio protein, which is known as an RNA-binding factor, and can encode RNA to be recognized, binding thereof to DNA has not been reported (Non-patent documents 8 and 9).
However, in the process of investigating properties of various kinds of PPR proteins, it became clear that it could be suggested that some types of the PPR proteins worked as DNA-binding factors.
The wheat p63 is a PPR protein having 9 PPR motifs, and it is suggested by gel shift assay that it binds to DNA in a sequence-specific manner (Non-patent document 10).
The GUN1 protein of Arabidopsis thaliana has 11 PPR motifs, and it is suggested by pull down assay that it binds with DNA (Non-patent document 11).
It has been demonstrated by run-on assay that the Arabidopsis thaliana pTac2 (protein having 15 PPR motifs, Non-patent document 12) and Arabidopsis thaliana DG1 (protein having 10 PPR motifs, Non-patent document 12) directly participate in transcription for generating RNA by using DNA as a template, and they are considered to bind to DNA.
An Arabidopsis thaliana strain deficient in the gene of GRP23 (protein having 11 PPR motifs, Non-patent document 14) shows the phenotype of embryonal death. It has been demonstrated that this protein physically interacts with the major subunit of the eukaryotic RNA transcription polymerase 2, which is a DNA-dependent RNA transcription enzyme, and therefore it is considered that GRP23 also acts to bind to DNA.
However, bindings of these PPR proteins to DNA have been only indirectly suggested, and actual sequence-specific binding has not been fully verified. Moreover, even if such proteins bind with DNA, it is generally considered that binding of a protein and DNA, and binding of a protein and RNA are attained by different molecular mechanisms, and therefore what kind of sequence rule specifically exists, with which binding is attained, etc, are not even expected at all.